[IMP] point_of_sale: redirect POSBox image init script output to file

Useful when updates to the initialization scripts don't go as
planned. This leaves something to inspect.

The initialization script already automatically stops (because of 'set
-o errexit') but it was a bit tricky to actually see what went wrong
because scrollback in QEMU isn't great.

addons/point_of_sale/tools/posbox/overwrite_before_init/etc/rc.local

@@ -17,6 +17,6 @@ if [ "$_IP" ]; then
   printf "My IP address is %s\n" "$_IP"
 fi
 
-/etc/init_posbox_image.sh
+/etc/init_posbox_image.sh 2>&1 | tee /home/pi/init_posbox_image.log
 
 exit 0